Behavioral treatment of both exercise and eating habits appears to be the most effective way to produce lasting reductions in excess body fat. However, long term compliance to eating and exercise recommendations is still poor in the majority of patients. Because of the many benefits of exercise for the obese, one emphasis of this study will be to explore more effective ways to increase exercise. Previous studies have used external reward structures to increase exercise. However, most habitual excercisers seem to be motivated by the intrinsic rewards of aerobic fitness. This study will combine the external reward approach with one which emphasizes intrinsic reinforcement, based on guided experiential learning of the psychological benefits of aerobic exercise. This approach will be used for the exercise portion of the following treatments: 1) Exercise and diet behavior modification; 2) Exercise modification only; 3) Diet modification only; and 4) No treatment control. In the context of these behavioral treatments for obesity, this study will explore the cognitive-behavioral processes associated with successful and unsuccessful attempts to change eating and exercise patterns. These replace and maintenance processes will be studied over a one-year period. Neither long-term behavior change processes nor use of intrinsic reinforcement has received much attentic in previous research. This study will examine such processes and how they are affected by different treatment approaches. In addition to behavioral outcome and process analyses, this study will also use as physiological outcome measures cholesterol and HDL ratio, weight, and estimated maximal oxygen utilization. This will enable an objective estimation of risk reduction. Overall, the information derived should be useful in helping to develop strategies for exercise and diet change in the obese and other populations.